i have a crapload of car relays, and i was wondering, i know the coil can only handle the rated voltage, like operating the coil at 12v for instance, but im curious, since these are high amperage relays, would the contact switch be able to handle 120v ac mains to control lights and sockets ??

reason i ask, is im making a teleduino controlled home system, i want to be able to turn on and off our air conditioners and such, like for instance, when we are out in the summer and come home, have the a/c off while out, and about 20minutes before coming home turn them on, but since a/c's use high amps, i cant seem to find a relay powerful enough to handle it, all the arduino relay boards ive seen are either 10a, or 15a,

but again,will the contact switch terminals handle 120v ac.. ?thats my concern, im no electrician, but i mess with this stuff, and i just dont want something failing and having to explain to landlord why the house went poof

Most large ac units have internal relays/contactors operated via a 24v coil. You may want to look at a relay that will also switch the control 24v coil in parallel to operate the main relays/contactors.

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but again,will the contact switch terminals handle 120v ac.. ?thats my concern, im no electrician, but i mess with this stuff, and i just dont want something failing and having to explain to landlord why the house went poof

I suspect most 'automotive' relays are not rated for 120vac operation and unless they were I wouldn't use them as such. I was just responding to the statement:

I suspect most 'automotive' relays are not rated for 120vac operation and unless they were I wouldn't use them as such. I was just responding to the statement:

From what I have seen about relay specs, the contacts are rated for both AC and DC - generally for AC, you get a higher current allowance on the contacts than you do for DC (and generally the DC is at a much lower voltage). I think part of it has to do with arc-over possibility from DC vs AC (which is alternating thru a 0 volt area, and thus the arc-over is less likely). That isn't to say that all DC relays could be used for AC - but there is a strong chance that a good quality DC relay will work fine for AC switching.

My wife works as a barista at a combo coffee and ice-cream shop, and her boss is cheap. I get to play "appliance repairman" on occasion, because her boss buys second-hand commercial units that have no service contracts or such, and cost an arm-and-a-leg to have serviced or repaired - or cost twice that amount to buy new. One day, my wife came home with the business' waffle cone maker. This is essentially a waffle maker that has two griddles, you put some batter in, close it, hit a button, it counts down and you have a waffle made that is rolled into a cone, cooled - and there you have something to put your ice cream in.

It was broken - wasn't heating up at all. I took it apart, and saw that there was a small micro-controller unit hooked up to the LED countdown system, plus a bit of circuitry connected via wires to a transformer that converted the AC coming into the box to 12 VAC, which was then rectified on the microcontroller board, and probably ultimately turned into 5 volts for the controller. Other wires left the board and went into a relay, which I could see had the AC wires across it that went to the heating coils of the griddles. So - the controller heats it up, and regulates the temperature of the griddle by switching it on and off (there was also a temperature sensor in there as well - probably a thermocouple of some sort).

I could see that the relay was dead burnt out (it was literally charred). The company that made the griddle wanted about $800.00 to repair this machine. This thing didn't even have $100.00 in parts or labor in (talk about making a killing - man, manufacture commercial cooking equipment, and you're set for life!). So - to repair it, all I needed was a relay...

Well - it was a 12VDC Omron relay - I looked it up, it needed 20A contacts. Since it was late-evening, nothing was open to buy parts from (Fry's Electronics was already closed), so I figured, what the heck - a cheap Bosch relay from AutoZone should work. I went down there, and picked up a 40A unit meant for switching something or another (blister pack special for auxilary usage). It had a screw mount, which was perfect because the original Omron relay was screwed to the sheet metal chassis of the waffle cone maker. I got it all hooked back up, we turned it on - and the thing heated up just like it was made to!

We ended up taking it back to the shop that night, and ran a batch of waffle mix through it to make more cones (well, in this case, it was a bunch of waffle bowls). It made them perfectly.

A few days later, my wife brought me a hundred bucks from her boss, and her boss was really grateful. That was several months back; that machine gets used every day, and it still works great. That relay will probably outlast the machine (even so, her boss only got the 5-5 warranty from me - 5 minutes or 5 cones, whichever comes first).

Anecdotal, sure - I wouldn't trust such a fix to get me to the moon or something - but at the same time, it was cheaper than the other option (and faster than getting the Omron replacement relay, which I did find on DigiKey, and it wasn't that expensive - but the business needed the machine working -today-, not a week or more later)...

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

Automotive starter relays are probably made for heavy current. Radio Shack list relays for up to 40A. I've always liked trying to fix broken things myself just to see if I can do it (if the repair will be fairly expensive).

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